1. Field of the Invention
The present invention relates to an ethylene-vinyl alcohol copolymer composition that comprises a powder of an ethylene-vinyl alcohol copolymer and inorganic particles, and to a powder coating composition comprising the ethylene-vinyl alcohol copolymer composition. The invention also relates to a method for producing such an ethylene-vinyl alcohol copolymer composition.
2. Discussion of the Background
Polyethylene, polyamide or epoxy resin powders have been used as resins for powder coatings to prevent metals or metal substrates from becoming corroded, and to protect metals from exposure to solvents. In particular, it is desirable to minimize the use of solvent-type coating compositions in order to reduce the load of solvents in the environment. Consequently, powder coating compositions are considered to be useful in increasing productivity and reducing costs.
However, acrylic, epoxy, polyamide or polyethylene resin powders are principally used as powder coating resins for preventing metals from becoming rusted or corroded. Since the gas-barrier properties and/or the chemical resistance of these resins are insufficient, films prepared from conventional resins must be very thick. In addition, since the interlayer adhesion of the resin film coating to the surface of metal is insufficient, the metal surface must be pre-treated with a primer or the like before being coated.
To solve these problems, Japanese Patent Laid-Open No. 115472/1991 discloses a method of using an ethylene-vinyl alcohol copolymer (hereinafter abbreviated as EVOH) powder for a powder coating resin.
In addition, Japanese Patent Laid-Open No. 241537/1997 discloses a powder coating resin composition that comprises 100 parts by weight of a saponified ethylene-vinyl acetate copolymer having an ethylene content of from 15 to 70 mol % and a degree of saponification of the vinyl acetate component of at least 80 mol % and having a melt index of from 1 to 100 g/10 min, from 0.0001 to 1 part by weight of an acid compound, and from 0.1 to 1 part by weight of a low-boiling-point compound having a boiling point of not higher than 200° C. This improves the uniformity of the thickness of the resin-coating layer of articles coated with the EVOH powder and improves the impact resistance of the surface of the coating layer. Japanese Patent Laid-Open No. 241537/1997 indicates that water is a preferred low-boiling point compound having a boiling point of not higher than 200° C., because it is safe and has a low environmental impact. Furthermore, when the amount of the low-boiling point compound having a boiling point of not higher than 200° C. in the composition is greater than 1 part by weight, and when the composition is applied to substrates by means of a flame spray coating process, in which a rapid temperature increase is inevitable, then the surface of the coating layer formed is greatly roughened.
However, EVOH is a polymer of high hydrophilicity and it absorbs water. In particular, compared with ordinary commercially available EVOH pellets, EVOH powder having a particle size less than 850 μm has a large surface area per the unit volume, and therefore it absorbs water readily. (EVOH powder having a particle size of less than 850 μm referred to herein is an EVOH powder that contains at least 80% by weight of particles capable of passing through a sieve having a nominal size of 850 μm as stipulated in JIS Z-8801.)
Therefore, when an EVOH powder having such a small particle size is left in an open space for a long period of time, it will absorb ambient water, and may contain at least 1 part by weight (for example, from 1 to 3 parts by weight or so) of water relative to 100 parts by weight of EVOH. Accordingly, if an EVOH powder which has been left exposed to ambient moisture for a long period of time is used for powder coating compositions, the powder is preferably dried before use.
However, drying the EVOH powder involves additional cost and is troublesome. In particular, when only a small amount of EVOH powder is needed, the EVOH powder must be repeatedly dried whenever it is used, which will cause thermal degradation of the EVOH powder. In addition, when conventional EVOH powder is used for coating substrates continuously over a long period of time in high-humidity conditions, the water content of the EVOH powder gradually increases, and, as a result, the uniformity of the coating film formed is often poor (that is, the coating film becomes less smooth and glossy). Therefore, depending on the working environment (especially in high-humidity conditions), it is not always easy to continuously coat substrates with a coating composition containing EVOH powder, over a long period of time. Thus, it is desirable to develop a powder coating method in which a powder composition comprising even an EVOH powder that contains one or more parts by weight of water relative to 100 parts by weight of EVOH, can form coatings having high uniformity.